Exothermic reactions often take place in catalytic conversions accomplished by passing a process stream of gaseous raw material through a bed of a solid catalyst under convenient pressure and temperature conditions. The synthesis of ammonia or methanol and the Fischer-Tropsch synthesis are important industrial examples of this kind of processes.
The heat of reaction evolved in exothermic reactions increases the temperature of the process stream and the catalyst and this often results in deterioration of the catalyst performance and in reduction of the concentration of intended products because the overall reaction rate responds vigorously to changes of the temperature and distribution of the temperature in the catalyst layer or bed. In case of reversible exothermic reactions, the equilibrium concentration of the product is declining with increasing temperatures, thus getting more unfavourable at high temperatures.
The temperature profile in the catalyst layer during exothermic reactions depends not only on the rate of the evolution of heat of reaction but also on the method of removing heat from the catalyst bed to avoid excessive elevation of the temperature of the reacting gaseous material and the catalyst.
Substantially three different methods are used for removing the heat of reaction from the catalyst bed: direct cooling by mixing with a cold feed gas; indirect cooling by heat exchangers; and using cooling tubes in the catalyst bed.
A method which is frequently used at present for removing excessive heat is heat exchange between a high temperature gas leaving the catalyst layer and a cold feed (synthesis) gas, thereby elevating the temperature of the feed gas to a level necessary for initiating the reaction. Gas-gas heat exchanging units are thereby usually disposed centrally in or after one or more catalyst beds. However, in this manner only minor parts of the catalyst bed will be at optimum temperature; and consequently, by this method large parts of the catalyst bed suffer from insufficient temperature control.
To remove heat of reaction more uniformly from the entire catalyst bed, there has been designed reaction vessels in the prior art which are provided with cooling tubes which extend through different regions of the catalyst bed. Thereby excessive heat is transferred to a cold feed gas or to an external cooling medium. The gas or medium which enters the cooling tubes extending through the catalyst bed absorbs the heat evolved in the reaction. As the temperature of the reacting gas in the catalyst bed increases, the temperature difference between the reacting gas and the cooling tubes will increase and the temperature will thereby in some regions of the catalyst bed exceed the temperature for a maximum reaction rate. Therefore, the temperature control is sluggish and temperature oscillations around the cooling tubes dampen out very slowly. Reactors based on such a design are the known counter-current axial flow ammonia converter of the Tennessee Valley Authority type (TVA) as described in Industry. Engn. Chem. 45 (1953), 1242 and the co-current axial flow ammonia converter of the Nitrogen Engineering Corporation, mentioned in Br. Chem. Eng. 8 (1963), 171.
Cooling of a radial flow reactor constitutes a special problem: in order to carry out the cooling surface has to be kept constant throughout the height of the catalyst bed although it may vary with the radial position in the bed.
A method disclosed in U.S. patent application Ser. No. 4,321,234 for removing the heat of reaction in a radial flow reactor is to evaporate a liquid under a convenient pressure by passing a liquid cooling medium through cooling tubes. A cooling medium in the form of a rising stream is introduced and distributed via a system of distribution tubes into a number of second distribution tubes and then to a large number of cooling tubes connected to the second distribution tubes controlling the temperature inside the catalyst bed.
However, the large number of tubes connected to each other and the necessary piping results in a complicated network construction which renders the filling or refilling of catalyst charges a troublesome operation. A serious disadvantage of this method is even the risk of poisoning the catalyst by cooling media in case of leaks in the tube or piping system.
Another disadvantage of the method disclosed in the above U.S. Patent is the demand of external preheating of the feed gas to a temperature required for initiating the reaction in the catalyst bed and adjusting the boiling point of the cooling medium to a level which is just below or about the temperature inside the catalyst bed and which is determined by the kind of the conversion reaction.